Effects of applied currents on turtle cones in darkness and during the photoresponse.

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Abstract
The voltage changes, produced by pulses of current of various intensities and polarities into turtle [Pseudemys scripta elegans and Emys orbicularis] cones, were examined both in darkness and during the photoresponse. Depolarizing pulses of current produced voltage deflexions which, during the photoresponse, were increased to a greater extent than those produced by pulses of current of the same intensity and opposite polarity. Voltage-current relationships were measured both in darkness and during illumination. During illumination they displayed a non-linear behavior, the curve being steeper for positive than for negative currents. These non-linearities were greatly attenuated following exposure of the retina to a solution containing 5 mM CoCl2 which is known to block synaptic transmission. The effects of CoCl2 were readily reversible when the retina was returned to normal conditions. By comparing the effects produced by pulses of current injected into individual cones with those observed in horizontal cells following stimulation with a radial current, it was observed that the non-linear behavior of the cone membrane during the photoresponse can be, at least in part, accounted for by events occurring in post-synaptic elements (i.e., horizontal cells).